The present invention relates to the production of photographs, photocopies, or other fixed images. More specifically, it is an imaging system which employs a coated substrate which contains a chromogenic material and a microencapsulated photosensitive composition. In the most preferred embodiment of this invention the chromogenic material and photosensitive composition are in the same microcapsules.
Several known imaging systems employ photosensitive encapsulates. One such imaging system which has significant advantages over all previously known ones is referred to generically as the Sanders process. This process employs a coating composition which is usually applied to a substrate. The coating includes a photosensitive composition which is encapsulated and a chromogenic material which may or may not be within the microcapsules. ("Encapsulated" is used herein to refer to both open phase systems in which the photosensitive composition is dispersed as droplets throughout a dispersing medium and systems in which there is a discrete capsular wall.) The microcapsules generally have a mean diameter of 1 to 25 microns. Images are formed by imagewise exposure of the coating composition to actinic radiation. ("Actinic radiation" is used herein to designate the entire spectrum of electromagnetic radiation.) "Imagewise" exposure means that radiation is applied in a pattern such that areas which are to be dark receive the most radiation while areas which are to be light receive little or no radiation, or vice versa. This can be accomplished, for example, by placing a stencil between the radiation source and the coating. Exposure can be through either direct transmission or reflection imaging.
After exposure, the microcapsules, or at least those in the image areas, can be ruptured by calendering or other suitable means. In the case of a photohardenable photosensitive composition, the viscosity of the photosensitive composition is increased substantially upon exposure to actinic radiation, through mechanisms such as cross-linking or simple polymerization. Therefore, when the capsules are broken, the photosensitive composition which received a strong exposure will flow very little, if at all, while the unexposed or weakly exposed photosensitive composition can flow relatively freely. As a direct result, the chromogenic material reacts imagewise with the developer according to the degree of exposure to form a color in the form of the desired image. This can occur several different ways.
In one embodiment, the chromogenic material is encapsulated with the photosensitive composition. Outside the capsules, a developer is contained in the coating, which is applied to a substrate. When the capsules are ruptured, the chormogenic material is available to flow, but its movement from the exposed capsules is restricted by the increased viscosity of the photosensitive compositions in those capsules. As a result, the accessibility of chromogenic material to developer depends on the exposure received locally. The developer and chromogenic material react according to the exposure to form the desired image. When this embodiment of a coating composition is applied to a substrate, the result is a self-contained imaging sheet.
In another embodiment, the photosensitive composition is encapsulated and the chromogenic material is within the coating inside or outside the capsules. A developer can be located as a separate layer from the chromogenic material in the coating, or can be on a separate substrate altogether. In the former situation, capsule rupture releases imagewise the photosensitive composition. The chromogenic material now reacts with the developer to form a color generally in the form of an image. In the latter situation, the two substrates are superimposed during capsule rupture so that the dissolved chromogenic material flows onto the developer sheet and reacts imagewise there.
In an alternative embodiment, the photosensitive composition can be a high viscosity substance which depolymerizes upon exposure to actinic radiation. In that case, the chromogenic material located in or near exposed capsules, instead of unexposed ones, is made accessible to the developer. This changes the imaging system from a positive one to a negative one.
The photosensitive composition includes a photoinitiator and a substance which undergoes a change in viscosity upon exposure to light in the presence of the photoinitiator. The substance may be a monomer, dimer, or oligomer which is polymerized to a higher molecular weight compound or it may be a polymer which is cross-linked. Alternatively it may be a compound which is depolymerized or otherwise lysed upon exposure. Radiation curable materials that are often used are materials curable by free radical initiated chain propagated addition polymerization or ionic polymerization.
Representative radiation curable materials are ethylenically unsaturated organic compounds. These compounds contain at least one terminal ethylenic group per molecule. Typically they are liquid at room temperature and can also double as a carrier oil for the chromogenic material in the internal phase. A preferred group of radiation curable materials is ethylenically unsaturated compounds having two or more terminal ethylenic groups per molecule. Representative examples of these compounds include ethylenically unsaturated acid esters of polyhydric alcohols such as trimethylol propane triacrylate or trimethacrylate, acrylate prepolymers derived from the partial reaction of pentaerythritol with acrylic or methacrylic acid or acrylic or methacrylic acid esters; isocyanate modified acrylate, methacrylic and itaconic acid esters of polyhydric alcohols, etc.
Some typical examples of photosoftenable materials useful in other embodiments are photolysable compounds such as certain diazonium compounds, 3-oximino-2-butanone methacrylate which undergoes main chain scission upon U.V. exposure and poly 4'-alkyl acylo-phenones.
Various photoinitiators are used. These compounds absorb the exposure radiation and generate a free radical alone or in conjunction with a sensitizer. Suitable photoinitiators include .alpha.-alkoxy phenyl ketones, Michler's ketone, O-acylated .alpha.-oximinoketones, polycyclic quinones, benzophenones and substituted benzophenones, xanthones, thioxanthones, halogenated compounds such as chlorosulfonyl and chloromethyl polynuclear aromatic compounds, chlorosulfonyl and chloromethyl heterocyclic compounds, chlorosulfonyl and chloromethyl benzophenones and fluorenones, haloalkanes, .alpha.-halo-.alpha.-phenylacetophenones; photoreducible dye-reducing agent redox couples, halogenated paraffins (e.g., brominated or chlorinated paraffin) benzoin alkyl ethers, etc.
The above-described embodiments are only a few of the possible variations on the Sanders process. The means and methods of each comprise an imaging system which has substantial practical, commerical and functional advantages over prior art imaging systems. Various aspects of this process are disclosed in the following co-pending applications, which are incorporated herein by reference: Ser. No. 302,356 filed Nov. 12, 1981 now U.S. Pat. No. 4,399,209; Ser. No. 320,643 filed Nov. 12, 1981 now U.S. Pat. No. 4,440,846; and Ser. No. 339,917 filed Jan. 18, 1982.
After exposure and capsule rupture, the oil phase (i.e., the photosensitive composition and the chromogenic material) migrates to the developer layer which is on the same substrate as the microcapsules in a self-contained imaging sheet and on a separate substrate in a transfer sheet.
Some of the color formed is retained in the photosensitive oil phase in a mobile solution after image formation. This is particularly true where large amounts of heavier photosensitive oils form the internal phase. If the photosensitive composition does not quickly react to visible light following capsule rupture and image formation, in some cases the image may bleed down into and across the imaging sheet, blurring and reducing the intensity of the image. This effect is sometimes referred to as "feathering".